How to edit Parameters for an Object after it is created?
Blender does not have a construction history. You can only edit the parameters of the last action, nothing earlier than that.
To edit the parameters of the last action you can either use F9, you can go to Edit &gt; Adjust Last Operation or open up the panel on the bottom left of the 3d Viewport.
 
If you want to modify the last tool you have to do it before using another tool.

In Blender 2.7x you can use F6 or the Operator Tab in the Tool Panel T to edit the parameters of the last action.